this field has already been the subject of much study, described in particular in U.S. patents U.S. Pat. No. 3,538,173, U.S. Pat. No. 3,553,276, U.S. Pat. No. 4,128,591 and U.S. Pat. No. 4,255,606. In known processes for isomerising aromatic compounds containing eight carbon atoms, a feed which is depleted in para-xylene with respect to the thermodynamic equilibrium of the mixture and which is rich in ethylbenzene (with respect to that same mixture at thermodynamic equilibrium) is introduced into a reactor containing at least one catalyst. That feed enters the reactor where the temperature and pressure conditions are suitable for obtaining a composition of aromatic compounds containing eight carbon atoms at the outlet from that reactor which is as close as possible to the composition of that mixture at thermodynamic equilibrium. Other documents describe processes for isomerising aromatic compounds containing eight carbon atoms in which the feed to be treated is rich in xylenes and depleted in ethylbenzene. The term "depleted in para-xylene" and "rich in ethylbenzene" as used in the present description mean that the para-xylene content is substantially lower and the ethylbenzene content is substantially higher than that of a mixture at thermodynamic equilibrium, under the temperature and pressure conditions under consideration (the mixture being constituted by meta-xylene, ortho-xylene, para-xylene and ethylbenzene).
From that mixture, the para-xylene and often the ortho-xylene are separated out since they are the isomers which are sought as they are of importance, in particular to the synthetic fibre industry. The meta-xylene and possibly the ethylbenzene can then be recycled to the isomerisation reactor inlet so as to increase the production of para-xylene and ortho-xylene.
The reaction for isomerising aromatic compounds containing eight carbon atoms per molecule, however, encounters a number of problems caused by secondary reactions. Thus in addition to the principal isomerisation reaction, hydrogenation reactions are observed: hydrogenation of aromatic compounds to naphthenes, also naphthene ring opening reactions which lead to the formation of paraffins containing at most the same number of carbon atoms per molecule as the naphthenes from which they originate. Those paraffins can undergo cracking reactions which lead to the formation of light paraffins typically containing 3 to 5 carbon atoms per molecule. Aromatic compounds undergo dismutation and transalkylation reactions which, in the case of aromatic compounds containing eight carbon atoms, lead to the production of benzene, toluene, aromatic compounds containing 9 carbon atoms per molecule (for example trimethylbenzenes) and heavier aromatic compounds.
The aggregate of such secondary reactions substantially and deleteriously affects the yields of desired products.
The quantity of secondary products formed (essentially naphthenes, paraffins, benzene, toluene, and aromatic compounds containing 9 or 10 carbon atoms per molecule) depends on the nature of the catalyst and the operating conditions of the isomerisation reactor (temperature, partial pressures of hydrogen and hydrocarbons).
An examination of the prior art documents reveals that recycling certain constituents contained in the effluent from the isomerisation reactor to the inlet of that reactor to minimise the production of secondary products has been envisaged.
As an example, U.S. Pat. No. 3,553,276, U.S. Pat. No. 3,558,173 and U.S. Pat. No. 4,255,606 recommend adding certain produces to the feed to be treated to reduce the loss due to secondary products.
U.S. Pat. No. 3,553,276 describes an apparatus in which toluene is recycled such that the concentration of toluene is kept at double the concentration which would be obtained without that recycling.
U.S. Pat. No. 3,558,173 describes recycling naphthenes containing eight carbon atoms produced by hydrogenation of the corresponding aromatic compounds to the reactor inlet.
In the description of U.S. Pat. No. 4,255,606, 1% to 10% by weight with respect to the total feed of an aliphatic hydrocarbon containing at least 5 carbon atoms per molecule is introduced into the reaction zone with or without adding toluene. This addition can be effected by recycling. The hydrocarbon introduced can also be a precursor of n-pentane.